CHEM2210 Chapter Notes - Chapter 9: Centrosymmetry, Coordination Complex, Laporte Rule
More on Temperature Dependence:
Example Question:
1. The temperature dependence of transition metal complex spectra can
reveal the mechanism via which d-d transitions gain intensity. In the
case where transitions do change intensity and position, the
mechanism is thought to be associated with vibrational motion of the
complex i.e. dynamic symmetry breaking.
• Ground and excited state vibrational states for an optical transition
• Excitation of the electronic transition associated with the lowest
vibrational state appears to the blue (higher energy) than that of higher
vibrational states.
• As these higher states are populated at higher temperatures, d-d
absorption bands tend to shift to the red at higher temperatures, for
centrosymmetric molecules.
• The band can gain intensity due to better overlap ground and excited
vibrational states (higher Frank Condon factors).
Electronic transitions → increasing intensity:
i. spin-allowed d-d transitions in a tetrahedral complex
ii. spin-allowed d-d transitions in an octahedral complex
iii. spin-forbidden d-d transitions in an octahedral complex
find more resources at oneclass.com
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Document Summary
Example question: the temperature dependence of transition metal complex spectra can reveal the mechanism via which d-d transitions gain intensity. The band can gain intensity due to better overlap ground and excited vibrational states (higher frank condon factors). Electronic transitions increasing intensity: spin-allowed d-d transitions in a tetrahedral complex spin-allowed d-d transitions in an octahedral complex spin-forbidden d-d transitions in an octahedral complex charge-transfer transitions in an octahedral complex. Spin-forbidden d-d transitions in octahedral complexes are both spin and laporte forbidden. Spin-allowed transitions in octahedral complexes are laporte forbidden. Spin-allowed transitions in tetrahedral complexes relax the laporte rule because the absence of a centre of symmetry allows mixing of d and p orbitals. Charge-transfer transitions are both spin-allowed and laporte allowed. The laporte rule is a spectroscopic selection rule that only applies to centrosymmetric molecules (those with an inversion centre) and atoms.
